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Various ideas and attempts to cross build ports:
trhodes' bsd.crossbuild.mk
distcc, NFS, and lots of hardware
http://blog.chris.tylers.info/index.php?/archives/249-Fedora-ARM-PandaStack.html
Use lots of target hardware in a cluster connected to a NFS server. Farm out the work across the cluster.
distcc, NFS, and an emulator
Another possibility is to run the compiler on the host, but everything else in the emulator. There are a few ways of doing this. The simplest is to use distcc, but that still does the preprocessing in the emulator, which is not ideal, but faster than doing the entire compile there. If they share a filesystem (e.g. an NFS server in the host) then you could probably replace cc, c++, and so on in the emulated environment with shell scripts that ran the cross compiler on the host (just needs to pass the arguments down a socket, so a wrapper around netcat would probably work).
"User Mode" QEMU and endian independent UFS
This idea comes from the paper: http://www.mewburn.net/luke/papers/build.sh.pdf
Krister Walfridsson has proposed building the packages natively in an emulator, using optimizations such as only emulating user-mode, and capturing system calls and running those natively (after manipulating the arguments). An enhancement to that is to detect if an emulated program is gcc (for example), and invoking the host's cross-compiler natively for that case. The initial progress looks very promising, with an arm emulator compiling six times faster than the native platform.